Smart Quality: Uniform running at low speeds: How it works with cogging torque compensation

Jürgen Dlugosch, Baumüller Nürnberg GmbH

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Baumüller Smart Value
Get more out of your drive technology with IoT: Baumüller Smart Value is all about the added value that smart drive technology gives mechanical engineers and machine operators. Baumüller Smart Value enhances the scope of our servo drives: It takes existing data, such as power, current, torque, speed and position, and provides users with important additional information (e.g. machine operating status) or decisions are made directly in the drive to prevent machine failures. The same can be said of the associated design software, which ensures that the motor / controller combination is optimally dimensioned for the respective application and the controller is parameterized. In addition, the design can be immediately verified and optimized on the digital twin using simulation technology.

Index

What is cogging torque?

Cogging torque is a physical phenomenon in permanent magnet electric motors. The air gap varies, depending on the position of the rotor, so that the magnetic resistance is formed more or less well. The consequence is torque fluctuations and jerky running, particularly at low speeds.

A detailed description of this effect can be found here:
Basics: What is the cogging torque?

The problem

For certain applications with high smooth running requirements, without a cogging torque compensation this effect can affect the production process and have a negative influence on the quality of the end product. One example of this is the production of films on calender machines, on which high-quality films are produced at very low speeds. A calender must have very uniform running even at low speeds, to achieve the best results. Particularly in the production of plastic films, wave movements (ripple) in the speed cause thickness differences in the material, which make the end product unusable.

The solution

One possible solution is cogging torque-reduced motors, such as the DSH1 high-precision servo motors made by Baumüller. If such high-precision motors cannot be used for performance reasons, then Baumüller offers the “cogging torque compensation” function integrated in the drive.

The functionality is available in the firmware. The ProDrive analysis tool can be used to measure the real cogging torques and compensate for them in the servo drive. The cogging torques that occur are identified by an automatic measurement, the so-called teach-In, during commissioning. The controller is notified at which point and with which strength the cogging torque will occur during normal operation. A position and angle-dependent current is then calculated in the drive controller, which counteracts the existing cogging torque. The precontrol signal at the setpoint of the torque-forming component of the current balances out the expected waviness. In this way, a uniform torque along the entire traversing range and maximum product quality are possible.

This function can be used, for example, to produce uniform and streak-free films by calendering.

Do you have any questions?

Jürgen Dlugosch
Senior Product Manager I Baumüller Nürnberg GmbH

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